1. Field of the Invention
The present invention relates to electronic communications and navigation systems and, more particularly, to methods and apparatus for efficiently multiplexing Code Division Multiple Access (CDMA) data streams into a constant-envelope composite signal using techniques based on majority logic.
2. Description of the Related Art
The revolution in digital communications that occurred in the latter half of the 20th century stemmed largely from the 1940's work of Claude Shannon at AT&T Bell Laboratories, as introduced in “A Mathematical Theory of Communication,” Bell Syst. Tech. J., vol. 27, pp. 379–423, 623–656, July–October 1948, the disclosure of which is incorporated herein by reference in its entirety. Shannon demonstrated the possibility of virtually error-free data communications on noisy channels as long as the transmission rate does not exceed a fundamental limit called the channel capacity. Prior to this realization, communication engineers relied solely on transmit power increase or brute-force redundancy (e.g., data rate shedding or bandwidth expansion) to improve the reliability of a link. One such form of redundancy is to transmit each data symbol an odd number of times, demodulate each symbol individually and decide in favor of the symbol value that occurs more frequently. This is simply decision by majority vote. In this use of majority voting, no combination of redundant data symbols occurs at the transmitter; the combination occurs as part of a decision process at the receiver. Fifty years of development from Shannon's theory have made obsolete this simple redundant encoding in favor of more efficient schemes that, unfortunately, function at the expense of increased decoding complexity.
The identical principal of redundancy has long been incorporated into design of digital logic circuits and subsystems. Triple modular redundancy (TMR) is a standard design practice for systems in which stringent availability and fault tolerance requirements exist. For example, the original design of the Space Shuttle made use of five guidance and control computers whose outputs were majority voted.
The majority vote principle has been recognized as a method to combine binary codes for at least 40 years; references dating to as early as 1962 are found, including: Easterling, M. F., “A Skin-Tracking Radar Experiment Involving the COURIER Satellite,” IRE Trans. SET, pp. 76–84, June 1962; Braverman, D. J., “A Discussion of Spread Spectrum Composite Codes,” Aerospace Corp., Report TDK-269, December 1963; Tausworthe, R. C., “Practical Design of Third-Order Phase-Locked Loops,” Jet Propulsion Lab., Calif. Inst. of Tech., (internal document) TR 900–450, pp. 19–30, April 1971; Spilker, J. J., “Digital Communications by Satellite,” Prentice-Hall Inc., Englewood Cliffs, N.J., pp. 600–603, 1977, the disclosures of which are incorporated herein by reference in their entireties. The purpose of code combination in the cited references, however, is to create a resulting single code with certain properties that make it desirable for use in ranging applications. Long pseudo-noise (PN) codes, consisting of random-appearing sequences of binary digits (1s and 0s) facilitate ranging by providing high resolution and freedom from the range ambiguities that may arise in periodic repetition of shorter sequences. More significantly, these effectively replace ranging based upon short, high-power pulses that require transmitters capable of very high peak power production.
Because acquisition (locating the time epoch) of long PN codes can be time consuming or can require massive parallel search capability, it is advantageous to build some substructure into the code that facilitates rapid acquisition without penalty in equipment or peak power requirements. In the aforementioned paper by Easterling, the idea of combining several PN sequences with relatively prime periods was advanced. The resulting code period is the product of the component periods. Easterling found that, if the combining is accomplished via majority vote, the epochs of each component code can be detected individually, and their coincidence used to determine the full code epoch. The effective power seen in each component code at a receiver is about 63% of its initial allocation due to the creation of undesired intermodulation components in the combination. In this application, there is no data communication associated with the codes; nor is there any end use of separating codes upon reception.
Two signals can be combined in a given bandwidth without mutual interference if placed on carriers in quadrature, i.e., the two independent, orthogonal sinusoidal components that exist at every frequency. When more than two signals are to occupy a given band without mutual interference, and each occupies the full available bandwidth, creative multiplexing techniques are needed to accomplish this, especially if the desired outcome is a signal having a low peak-to-average power ratio.
Interplex modulation was created with this need in mind, as reported by Butman, S. and U. Timor in “Interplex—An Efficient Multichannel PSK/PM Telemetry System,” IEEE Trans. Comm., June 1972, the disclosure of which is incorporated herein by reference in its entirety. An interplex signal has constant envelope, i.e., its instantaneous power level does not change with time. Using quadrature carriers, interplex modulation can combine any number of data-bearing, PN spread binary codes. The component signals may have any assigned power distribution. Interplex modulation is quite efficient in representing three components (efficiency is never less than 75% for any power allocation), but its efficiency drops rapidly as more signals are added, and is generally not useful for more than five components. Interplex modulation has connections to majority vote logic not recognized by its inventors. These connections have been exploited in a generalization called Intervote, in which elementary majority voting techniques are combined with interplex modulation, as described in U.S. patent application Ser. No. 09/963,669 by Cangiani et al., entitled “Methods and Apparatus for Generating a Constant Envelope Composite Signal,” filed Sep. 27, 2001, the disclosure of which is incorporated herein by reference in its entirety.
Though it had predecessors, the NAVSTAR Global Positioning System (GPS) became the world's first major navigation system relying on PN ranging and concomitant data transmission. In GPS, low-rate data signals that support the navigation mission are modulated onto PN ranging signals transmitted by a constellation of satellites in a manner almost transparent to the ranging function. Each satellite transmits no more than two distinct ranging codes in any one frequency band, and these codes are efficiently combined using quadrature multiplexing.
Under a program called GPS Modernization, the U.S. Government is studying techniques to enhance both the military and civilian utility of GPS. A possible outcome of this effort is the inclusion of three or four distinct PN codes in the signal transmitted by a satellite at one frequency. Means to employ majority logic techniques for multiplexing of these signals have been investigated by Spilker et al. in “Code Multiplexing via Majority Logic for GPS Modernization,” Proc. ION GPS-98, Nashville, Tenn., September 1998, the disclosure of which is incorporated herein by reference in its entirety.
CDMA transmission of voice/data in terrestrial cellular networks places more stringent requirements on CDMA than any of the prior applications. Traffic is two-way, and the number of codes per cell can be as many as 64 now and 128 or more in the future. Code channels have various functions: pilot, paging, synchronization, control, and traffic. To avoid the dominance of one or a few signals (the “near-far” problem), power control is required at both the subscriber and base station terminals. Because the user mix continually varies due to newly initiated and recently completed calls, user motion and cell-to-cell handoffs, power control is dynamic and rapid (on the scale of milliseconds). Difficult channel conditions are posed by multipath interference and signal obstruction in urban environments. Data rates and traffic loads are certain to increase far beyond anything presently seen. Security of data flowing through the network is needed for operations, maintenance, accurate billing and privacy. Although the primary function of the system is data transmission, there are a variety of reasons, e.g. E911, why determination of subscriber position will be a required, integrated function for all future mobile networks. Despite advances in signal combining and multiplexing techniques, there remains a need a system capable of multiplexing CDMA signals efficiently into a constant-envelope composite signal for application to this complex environment.